The role of cell-cell interactions and stage specific embryonic antigens in early mammalian embryogenesis will be studied using clonal murine embryonal carcinoma (EC) cell lines as a model system. The time at which EC cells in aggregates become irreversibly committed to differentiate into endoderm will be determined by constructing bilayered aggregates consisting of a core of EC cells surrounded by a rind of genetically marked exogenous endoderm cells; in the presence of the exogenous endoderm, endodermal differentiation of EC cells is prevented. To study the developmental functions of embryonic cell surface antigens, variant cell lines with altered cell surfaces will be isolated and characterized. Lectins and antibody (monoclonal SSEA-1) plus complement will be used to select the variants. Variant cell lines will be tested for their ability to differentiate in vitro and during tumor formation in vivo. The cell surface changes responsible for resistance to the cytotoxic effect of the lectins will be analyzed initially by separating labeled glycoproteins by gel electrophoresis. Finally, to help define cell functions required for EC cell differentiation, EC cell variants with altered developmental potential ("nullipotent" cells) will be analyzed by somatic cell hybridization. Complementation tests will be performed to determine if different "nullipotent" lines possess a common defect.